For a long time, the primary way of measuring visual acuity has been through the use of a Snellen visual acuity chart or "E-chart." This involves a test chart with a series of letters in the alphabet such as L, D, O, N, and E, which are very large at the top of the chart and decrease in size to the bottom of the chart. The person being tested stands 20 feet from the chart and covers one eye and is asked to read the letters on a given line on the chart. A person able to read the bottom line has a 20/15 visual acuity. A person who can read only the largest letters has a poor visual acuity of about 20/300. Both eyes are tested in this manner. The disadvantage of the Snellen visual acuity test is that it requires the cooperation of the subject being tested and the letters can be memorized. Young infants and children less than 3 years old cannot be tested with the "E" chart. The Snellen test will not detect strabismus (ocular alignment) or lens obstructions, such as cataracts or tumors.
Since the invention of the retinascope by Cuignot in 1873, it has been possible to assess the refractive state of the eye by observing the refraction of light rays from the retina. In retinascopy, the source light, point of observation, and the subject's eye are approximately collinear. The refractive state is generally determined by moving the source light and adjusting the power of a spectacle lens in front of the eye so as to cancel the apparent motion of the source light on the retina by making the peephole of the retinascope conjugate with it.
One prior art method is described in the University of Helsinki publication ACTA Ophthalmologia, volume 57, 1979. The article is entitled "A Simple Method for Screening of Children with Strabismus, Anisometropia or Ametropia by Simultaneous Photography of the Corneal and the Fundus Reflexes" by author Kari Kaakinen. This article presents a simple screening method for detecting strabismus, anisometropia, and ametropia in young children by simultaneous photography of the corneal and fundus reflexes with a conventional camera and flashlight. In this article Mr. Kaakinen demonstrated off-axis photo refraction for detecting strabismus and refractive error at one meter. One disadvantage to Mr. Kaakinen's system is that it does not appear to be able to detect refractive error between 2.0 diopters myopic and 1.3 diopters hyperopic. The system lacks sensitivity in the 0.75 to 1.3 diopter range. The retinal reflex fundus images are small because of the 55 millimeter lens. His system requires the use of cyclopegic agents to dilate the pupils for optimum results. This makes the system more difficult to use because cyclopegic agents can only be administered by a doctor. The Kaakinen System does not use an integral head positioning station.
A second prior art method is described in the Journal of the Optical Society of America, February 1974 (volume 64, No. 2). This article is entitled "Photorefraction: A Technique for Study of Refractive State at a Distance" by Howard C. Howland and Bradford Howland. In this article, Howard C. Howland and Bradford Howland demonstrated a special segmented photorefraction attachment with an integral fiber optic light guide mounted in the center of the attachment. This technique does not provide a picture of the fundus reflex but does show a star arm pattern that increases with dioptric error. The Howland and Howland system also uses a 55 millimeter lens. The system is more complex, does not use color film and does not have its head positioning station integrated with the camera system. No retinal reflex image analysis for strabismus, lens obstructions, or pupil size differences can be performed with this system.
An early experimental system was developed previously by the National Aeronautics and Space Administration at Marshall Space Flight Center, Alabama, working jointly with Electro-Optics Consultants, Inc., under a NASA contract. A 6.4 m phototype off-axis photorefractor system was developed under the NASA contract with E.O.C. This system used a 1000 mm f/11 Celestron.RTM. lens. The system had several deficiencies. (1) Image resolution was poor because of a combination of using the lens at its minimal focal point (short depth-of-field) and the use of ASA 400 high speed color slide film. (2) The camera station was independent of the head positioning station. (3) The system required a room long enough to provide 25 feet of unobstructed floor space and a permanent or semi-permanent set-up for operation, which required a special alignment procedure and made the system non-portable. (4) Photographs made with this system produced a large corneal reflection (a reflection of the flash from the outer surface of the eye), which took the form of a bright white circle obscuring up to 9% of the surface area (with a 6 mm pupil) of the retinal reflex image. (5) Refractive error indications were exaggerated; in some cases where minimum refractive error was indicated in the image, none could be found by ophthalmalogical examination.
The 6.4 m MFSC/EOC system also exhibited several problems. The depth-of-field of the 1000 mm lens focused at 6.4 m was approximately less than 13 mm. In order to obtain a decent film exposure using the f11 lens requires the use of high speed (ASA 400) color film which resulted in grainy images. The combination of lens focal distance and film resulted in minimally acceptable retinal reflex image quality. Another major deficiency of the system involved installation, requiring 7.6 m of unobstructed floor space, of two separate components, the camera and head positioning stations which required special alignment procedures to assure satisfactory system operation.
From the above it may be seen that none of the prior art systems provide a really quick and accurate way of detecting eye defects. Moreover, the Howland and Howland method is not readily capable of adequately screening infants or other noncommunicative persons for amblyopia, a conditon of poor vision even with the use of corrective glasses. Untreated, amblyopia can result in vision degradation, perhaps even total blindness. Children should be screened frequently for amblyopia, but screening programs have not been instituted in the United States due to a lack of a simple, reliable, fast, and relatively inexpensive method.
Therefore, it is an object of our invention to provide a method and apparatus for detecting eye defects which is simple, quick, accurate, and relatively inexpensive.
It is another object of our invention to provide a method and apparatus for detecting eye defects which does not require a permanent examination room or highly trained personnel to perform the screening operation.
It is a further object of our invention to provide a method and apparatus for detecting eye defects which does not require the subject of the examination to use cyclopegic agents or to do anything else except to look at a fixation light located near the lens of the camera.
It is a still further object of our invention to provide a method and apparatus for detecting eye defects which can isolate amblyopia and pre-amblyopic conditions and is also useful in detecting other eye diseases in their formative stages and in evaluating treatments.